In the hydrodesulfurization of petroleum residua, catalyst cost factors constitute a major problem. These cost factors are an aggregate of catalyst raw material and manufacturing costs, and the activity and deactivation rate of the catalysts. The problem is further aggravated by the fact that to date it has not been found commercially feasible to regenerate deactivated residua desulfurization catalysts, due principally to the deposition thereon during processing of metals such as vanadium and nickel, which are universally present in residual feedstocks.
Balancing all of the foregoing factors, the most cost-effective type of catalyst yet discovered for residual oil desulfurization is composed of minor proportions of cobalt and molybdenum dispersed in an alumina support. Early in the development of such catalysts, it become apparent that pore size distribution was an important factor. Obviously, at least a substantial proportion of the active surface area must be found in pores large enough to permit entry of the larger molecules of sulfur compounds found in residual feedstocks. This requirement would appear to call for pores having a diameter greater than about 30-50 angstroms. A complicating factor arose however when it was found that pores in the approximate diameter range of 100-2000 angstroms were detrimental, appearing to increase the deactivation rate of the catalyst. The explanation for this phenomenon appears to reside in the slow diffusion rate of the heavy feed molecules, coupled with the high ratio of volume to active surface area found in the larger pores. As a result, the feed molecules in the large pores tend to undergo thermal coking before they are effectively hydrogenated.
As a result of the foregoing considerations, several attempts have been made to optimize pore size distribution in residua desulfurization catalysts. For example, in U.S. Pat. No. 3,770,618, it is postulated that optimum deactivation rates depend upon providing a maximum pore volume in pores of 30-80 angstrom diameter, and a minimum in pores of greater than 100 angstoms. This pore distribution is achieved by compositing the alumina support with cobalt and molybdenum compounds, e.g. by impregnation, followed by drying and calcining at 1200.degree. F. In U.S. Pat. No. 3,853,788, a resid desulfurization catalyst is prepared by first precalcining the alumina-molybdena components in intimate admixture, then impregnating with a cobalt salt and again calcining. The final calcination is carried out at about 800.degree.-1300.degree. F, preferably about 1200.degree. F. Although the resulting pore structure is not disclosed, I have found that the catalysts so prepared display a pore size distribution similar to that described in the above noted U.S. Pat. No. 3,770,618.
A common feature in the preparation of Co--Mo--Al.sub.2 O.sub.3 resid desulfurization catalysts, which appears throughout the prior art and is exemplified by the foregoing patents, involves a final calcination at temperatures of about 900.degree.-1200.degree. F, or at most 1300.degree. F. For reasons which are not clear from the prior art, higher calcination temperatures have been eschewed; it may perphaps have been assumed that activity would thereby be reduced, inasmuch as that result has been known to ensue from the high-temperature calcination of Co--Mo--Al.sub.2 O.sub.3 catalysts used for desulfurizing distillate petroleum feedstocks.
I have now discovered however, that for purposes of desulfurizing resid feedstock, calcination temperatures in the range of about 1250.degree.-1400.degree. F, preferably 1325.degree.-1375.degree. F, give catalysts of distinctly higher activity and stability than do the conventional lower calcination temperatures. Moreover, this result is obtained notwithstanding the fact that the high-temperature calcination enlarges the 30-80 A diameter pores so that over 40%, and usually over 50% of the total pore volume of the catalyst is in pores of 75-100 A diameter, and less than 25% is in pores in the prior-art-recommended 30-70 A diameter range. It would in fact appear that this moderate enlargement of pores, in conjunction with some alteration in the nature of the active surface area brought about by high temperature calcination, actually is beneficial. However, calcination at temperatures above 1400.degree. F is detrimental, bringing about drastic reductions in surface area and mechanical strength as a result of extensive microcrystallization of aluminum molybdate and perhaps other phases. The pores are also drastically enlarged.
While I am unable to account with certainty for the improved results obtained herein, one possible explanation is that the relatively high calcination temperatures employed may tend to modify and anneal the active surface area so that some of the highly active cracking sites are moderated or eliminated, thus reducing coke forming reactions. Also, it is hypothesized that this unique type of active surface area may give increased hydrogenation rates versus cracking rates per unit of surface area in the larger pores, whereby somewhat larger pores become optimum, with resultant improved diffusion of feed molecules into and out of such pores. In any event the catalysts of this invention do provide improved activity and activity maintenance for the desulfurization of residual oils, as compared to the low-temperature-calcined, smaller pore catalysts of the type described in the above noted U.S. Pat. Nos. 3,770,618 and 3,853,788. Surprisingly however, the opposite appears to be true in the desulfurization of distillate oils; the catalysts of this invention have been found to be less active for that service than the prior art catalysts.